Title: Heteropolyacid-Transition Metal Interaction: Directing Structure and Templating Metal Organic Frameworks
Other Titles: Interactie tussen heteropolyzuren en transitiemetalen: structuur dirigerende werking en vorming van metaal organische roosters.
Authors: Bajpe, Sneha
Issue Date: 21-May-2012
Abstract: In this work, interaction mechanisms of heteropolyacids (HPAs) with transition metal ions Cu(II)and Cr(VI) were systematically investigated. Heteropolyacids are transition metal clusters withintriguing acid-base and redox properties. As a function of pH, heteropolyacids form differentlacunary species which in turn interact with transition metal ions. In this thesis, the resulting complexes have been used on the one hand to form a new material with interesting chemical properties and on the other hand as templates for the formation of functional porous materials.In 50% ethanol:H2O, the interaction of HPA with Cu cations enhances hydrolysis of monomericCu2+ species thereby inducing dimerization of Cu2+ on HPA. A series of pH measurements incombination with ESR and XANES conclusively prove the formation of dimeric copper on HPAsurface. With increasing Cu2+ concentrations, the proton activity in the solution is expectedto decrease, leading to an increase in the pH of the synthesis solution. Contrary to the expectations,a significant decrease in pH is observed, indicating protons are released by hydrolysis ofCu2+.The interaction of HPA with chromate reveals the formation of a key intermediate PW10 lacunaryKeggin species in solution. The quantitative conversion of Keggin-type Cu2+ heteropolyacidinto the K13[KP2W20O72] polyoxometalate proceeds via the formation of PW10 species. The major role played by chromate in the stabilization of this intermediate PW10 species is demonstrated by different spectroscopic techniques among which EXAFS and 183W NMR are crucial. 2:2:1 signals obtained from 183W NMR show the presence of a PW10 species with C2v symmetry which agrees well with the presence of an additional signal at -11.1 ppm in the 31P NMR spectrum. The -11.1 ppm signal grows slowly and then disappears after 5 days indicating thecomplete conversion of H3PW12O40 into K13[KP2W20O72] crystals. Several failed attempts tosynthesize the K13[KP2W20O72] crystals in the absence of chromate in synthesis solution revealed the significant role played by chromate in the formation of the intermediate PW10 speciesand subsequent precipitation into K13[KP2W20O72] crystals.Hydrolysis and associated dimerization of Cu2+ cations by the formation of HPA-Cu2+ complexesleads to the formation of HPA incorporated Cu3(BTC)2 ·3H2O-type metal organic framework(MOF) at room temperature. The HPA-Cu2+ complex infact behaves as a template for the formation of MOFs. The instantaneous precipitation of MOF framework is caused by systematicmolecular-level structure direction and templating by the HPA anions. The structure direction by HPA molecules to form Cu3(BTC)2 ·3H2O-type framework was studiedby 17O NMR, 31P NMR, 183W NMR, SAXS, NIR and dynamic light scattering techniques.All the results obtained from various spectroscopic methods were in agreement with the templating behaviour exhibited by HPA-Cu2+ complex during framework formation. The 183W NMR shift along with the appearance of several unidentified signals in the 17O NMR were interpreted as concrete evidence for the HPA-Cu2+ complexation and precipitation. The increase in particle size as evidenced by both SAXS and DLS confirmed the complexation and structure directing ability of the HPA-Cu2+ interaction mechanism.One of the striking features of the HPA incorporated Cu3(BTC)2 ·3H2O framework is its abilityto stabilize the MOF structure at high temperature under steam conditions. The room temperature synthesized HPA incorporated Cu3(BTC)2 ·3H2O framework remains stable up to 200 C while the empty Cu3(BTC)2 ·3H2O remains stable only up to 70 C. PXRD structural investigations revealed the stability of the room temperature synthesized HPA incorporated Cu3(BTC)2 ·3H2O framework. The reason for enhanced stability of the framework is the presence of HPAs in between the layers and hence preventing the organic linker molecules from collapsing into a different phase at higher temperatures. This is not the case with the empty Cu3(BTC)2 ·3H2O framework where there is no HPA to prevent structural change.The HPA incorporated Cu3(BTC)2 ·3H2O framework was grown on metallic Cu using it bothas a substrate and the source for Cu2+ cations. The crystals were grown in pure ethanol and50% ethanol:H2O solvents to identify the effect of solvent on orientation of the crystals. X-raydiffraction measurements revealed that the crystals grown in pure ethanol showed a preferred(100) orientation and the ones grown in 50% ethanol:H2O showed a preferred (111) orientation.This preferred orientation of crystals with change in solvent conditions enables the choice ofcrystal growth on thin films for different applications.
Publication status: published
KU Leuven publication type: TH
Appears in Collections:Centre for Surface Chemistry and Catalysis

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